Rainfed lowland rice varietal and seed management involves a complex system dealing with problems such as variety adoption, biodiversity, and the supply of good-quality seed. Participatory modeling of rainfed lowland rice varietal and seed management in lower northeast Thailand has been carried out to better understand the seed system and identify problems. Conceptual modeling was done through interinstitutional research team meetings, stakeholder analysis, surveying stratified random sampling of farmers and seed supply agents in Ubon Ratchathani Province, and conducting role-playing games (RPGs). The seed system was divided into three subsystems: farmers' decision-making related to rice varieties, farmers' management of seeds, and the whole existing seed supply system. A first RPG representing the first two subsystems was used at two different locations with 25 farmers. The initial findings from the RPG helped to validate and improve the conceptual model and provide a common understanding of farmers' rice varietal and seed management. Problems of limited access to or sharing of information about varieties and seed, the need for early-maturing varieties, and the scarcity of good-quality seeds were identified. A second RPG will deal with the whole seed supply system. A more comprehensive analysis of the RPG results with those of the farm survey will be done to improve the conceptual models, together with developing a multi-agent model representing the whole rainfed lowland rice seed system.